The present invention relates to a vacuum pump, particularly a turbomolecular pump or a multi-inlet turbomolecular pump.
Turbomolecular pumps comprise at least one rotor including a rotor arrangement with a plurality of rotor disks. Between the rotor disks, stator disks are arranged, being held by stator rings. The rotor arrangement is mounted on a fast-rotating rotor shaft. Turbomolecular pumps have an inlet on the suction side and an outlet on the pressure side. On the suction-side inlet, final pressures of possibly less than 1·10−10 mbar can be achieved. Frequently, the pressure-side pump connector has additional pre-vacuum pumps connected to it.
Multi-inlet pumps comprise at least one intermediate inlet in addition to a main inlet on the suction side. Usually, the rotor arrangement of a multi-inlet pump comprises two pump stages which can be formed e.g. as turbomolecular stages, with said intermediate inlet being arranged between these two pump stages. Often, a further pump stage, such as e.g. a Holweck stage, is provided behind the turbomolecular stages when viewed in the conveying direction. By use of multi-inlet pumps, different pressure levels can be generated at the main inlet and said at least one intermediate inlet.
Particularly in fast-rotating vacuum pumps, such as e.g. turbomolecular pumps and multi-inlet pumps, the support of the rotor shaft on the pressure side, i.e. in regions where no low pressures prevail, can be provided by means of electromagnetic bearings. In known vacuum pumps, the electromagnetic bearings provided for bearing support of the rotor shaft are operated in pressure ranges of up to 120 mbar. Further, it is known to use passive magnetic bearings for support of the rotor shaft in the high-vacuum region.
Electromagnetic bearings are not customarily used for the bearing support of a vacuum pump on the suction side, which is because of the low pressures in this region due to the circumstance that the coil bodies and sensor devices used therein are components with large surfaces and numerous cavities. Thus, because of the continuous outgassing, achieving the desired final pressure is not possible at all or is possible only with difficulties. Further, it is known to use passive magnetic bearings in the high-vacuum region.
For electromagnetic support of the whole rotor shaft, it has been proposed in DE 20 2005 019 644 to arrange the two electromagnetic bearings within a cartridge. Internally of said cartridge, the rotor shaft is arranged together with the bearings and the electric motor. In the direction of the pressure side, the cartridge is substantially open so that, within the cartridge, there will exist the atmospheric pressure or at least a relatively high pressure acting on the pressure side of the pump. The rotor shaft comprises a projection extending from out of the cartridge and carrying the rotor arrangement. Thus, the rotor arrangement is fastened to a cantilevered end of the shaft. Therefore, the constructional length of the pump is restricted. Further, the attachment of the rotor arrangement to the cantilevered shaft end will cause large forces at the bearing sites, entailing the necessity to install correspondingly complex electromagnetic bearings. Further, this constructional design is subject to massive restrictions due to the rotor-dynamic behavior, particularly because of low natural frequencies.
From U.S. Pat. No. 5,547,338, it is known to provide bearings of the type with turbulent fluidized bed. These bearings are basically different from electromagnetic bearings because bearings with turbulent fluidized bed are energized by the fields of the opposite permanent magnets. Exactly for the support of rotor shafts in turbomolecular pumps, bearing arrangements of this type are unfit for use because bearings with turbulent fluidized bed are very unstable. Further, if one were to use a bearing arrangement of this type, the ohmic losses in the conductive disks would cause a massive heat-up of the rotor until the magnetic fluxes would be sufficient for achieving the desired bearing effect. Further, this approach would cause a braking effect acting on the rotor and thus entail the need for an increased drive power.
It is an object of the invention to provide a vacuum pump, particularly a turbomolecular pump or a multi-inlet turbomolecular pump, wherein the bearing arrangements are improved.
The vacuum pump of the invention comprises a rotor shaft carrying a rotor arrangement, wherein the rotor arrangement can include, if required, a plurality of rotors or other suction or pumping devices. The rotor shaft is supported by—usually two—bearing arrangements, notably by a pressure-side bearing arrangement and a suction-side bearing arrangement. According to the invention, the suction-side bearing arrangement is arranged in a high-vacuum region and thus is exposed to low pressures. Further, according to the invention, the suction-side bearing arrangement is an electromagnetic bearing. A high vacuum is to be understood herein as a pressure below 10−3 bar, preferably less than 10−5 bar and most preferably less than 10−10 bar.
Particularly if the electromagnetic bearing is arranged in the region of very low pressures, as occurring on the suction side, i.e. in the inlet region of a turbomolecular pump, it is provided according to a particularly preferred embodiment that the coil of the electromagnetic bearing is arranged in a pressure-encapsulated recess. By arranging the coil in the pressure-encapsulated recess, it is safeguarded that the coils itself is not located directly in the high-vacuum region. This precludes the disadvantage that, due to the numerous cavities in the coil, the continuous outgassing will not or only with difficulties allow the final pressure to be reached. By the inventive provision of an electromagnetic bearing in the high-vacuum region, it is possible to support the rotor shaft in its end regions. Particularly, the rotor arrangement can be connected to the rotor shaft between the two bearings. An attachment of the rotor arrangement to a cantilevered arm of the shaft and thus a flying support of the rotor are not required anymore. Further, if also the pressure-side bearing arrangement is formed as an electromagnetic bearing, the resultant fully electromagnetic support of the shaft makes it possible to reach higher rotational speeds because the shaft can be given a very rigid design and the damping and stiffness at the bearing sites can be parametrized by software.